1. Field of the Invention
The present invention relates to a planar optical waveguide adapted to planar integrated optical parts and an optical module for use in the field of optical communication and a method of fabricating the planar optical waveguide, and more specifically, to a planar optical waveguide and a method of fabricating the planar optical waveguide capable of minimizing a propagation loss.
2. Discussion of Related Art
Optical passive components such as optical dividers, optical couplers, wavelength division multiplexers (WDM), optical filters, optical amplifiers, optical attenuators, and optical switches and optical active components such as light receiving devices and light emitting devices are employed to use optical signals, and an optical platform for integrating the optical passive components and the optical active components and various microwave-photonics optical modules using the optical platform are also used.
An optical waveguide is commonly applied to these components. Conventionally, a fiber optical component fabricated by fusing, coupling, and connecting an optical fiber has been used, but recently, a planar optical waveguide integrating an optical signal transmission path in a planar type using a semiconductor process is used. The planar optical waveguide becomes a basis arrangement in fabricating all planar optical components and optical modules.
FIGS. 1A to 1C are cross sectional views illustrating a method of fabricating a conventional planar optical waveguide.
Referring to FIG. 1A, a lower cladding layer 102 is formed on a substrate 101, and a core layer 103 having a refractive index larger than the lower cladding layer 102 is formed on the lower cladding layer 102. The lower cladding layer 102 and the core layer 103 are made of a silica layer, and elements such as, for example, GeO2, and P2O5 are added to the core layer 103 to have a higher refractive index than the lower cladding layer 102.
Referring to FIG. 1B, a metal pattern 104 is formed on the core layer 103, and then an exposed portion of the core layer 103 is etched through an etching process using the metal pattern 104 as a mask to form a core 103a. 
Referring to FIG. 1C, after the metal pattern 104 is removed, an upper cladding layer 105 having the same refractive index as the lower cladding layer 102 is formed on the entire surface of the resultant structure having the core 103a to complete the planar optical waveguide as shown in FIG. 2.
As described above, the conventional planar optical waveguide has an arrangement in which the lower and upper cladding layers 102 and 105 are formed around the core 103a with refractive indices lower than the core 103a, and an optical signal is guided through the core 103a by a principle that light is refracted and focused into a portion having a large refractive index. Therefore, elements such as GeO2, and P2O5 are added to the core layer 103 to increase the refractive index, thus generating a concentration gradient between the core 103a and the lower and upper cladding layers 102 and 105.
However, when the elements added to the core layer 103 are externally diffused due to a subsequent high temperature annealing process during the process of fabricating an optical waveguide, a difference of the refractive indices at an interface (A portion) between the core 103a and the lower and upper cladding layers 102 and 105 is reduced so that light guided through the core 103a is scattered at the interface (A portion) to cause a propagation loss.